1. Field of Invention
The present invention relates generally to computer disk drives and specifically to solutions to the problem of zone misses in optical disk drive systems that depend on a modified constant angular velocity (MCAV) method of data recording.
2. Prior Art
Optical disk drives are susceptible to seek errors because track densities are usually very high. After a head seek operation, it is often necessary to do a seek verification that quickly reads the present track address and computes any error distance that must be crossed to get to a target track. To read the track address, a reference synchronizing frequency used in the data readout circuitry must correspond to a data recording frequency for the recording zone in which the head(s) actually landed. When a zone miss occurs, the two frequencies will not match, because the actual zone differs from the target zone, and the data in the tracks cannot be read.
A popular method used in prior art magnetic disk drive systems to increase the recording capacity of optical disk and optomagnetic disk drive systems is a modified constant angular velocity (MCAV) method. This kind of method partitions the media into a number of concentric circular zones, and changes the data recording frequencies in the respective zones with the objective of making the line density of data recording uniform over the media's entire surface. This method has the advantage of allowing very high recording densities. It requires switching a reference synchronizing frequency to an appropriate data recording frequency for the zone in which the head(s) is/are positioned.
Moving a head to a target track in optical disk drive systems requires about ten times the precision of magnetic disk drives because of the much higher track densities. So most optical disk drives use a two-stage method that consists of a rough seek to moves the head(s) near a target track, followed by a fine seek to position the head(s) precisely on the target track. A typical program sequence, implemented as drive firmware, is as follows:
(1) reference a conversion table in a read only memory of a disk drive to find a target zone that contains a target track; PA1 (2) read a first track address to identify where a head is/are presently positioned and calculate a seek distance to the target track; PA1 (3) set a reference synchronizing frequency to the data recording frequency of the target zone; PA1 (4) move the head(s) to the vicinity of the target track with a rough seek; PA1 (5) read a second track address to identify where the head(s) has/have been positioned after the rough seek and calculate any error relative to the target track; and PA1 (6) do a fine seek based on the error. PA1 (1) increase (or decrease) the zone address setting of the disk drive circuit by one from the present address setting; PA1 (2) switch the reference synchronizing frequency to the data recording frequency of the new zone address; PA1 (3) re-read the track address; PA1 (4) repeat steps (1), (2), and (3) when the track address cannot be read; and PA1 (5) change the zone address to zero (or to the highest zone address) and return to step (2) when the zone address setting of the disk drive circuit reaches the highest zone address (or the address zero) of the media. PA1 switching a reference synchronizing frequency to the data recording frequency of an adjacent zone nearest to the seek target track; and PA1 switching a reference synchronizing frequency to the data recording frequency of an adjacent zone second nearest from the seek target track. PA1 defining a memory space in which to store a zone miss history; PA1 referencing the zone miss history as a basis for estimating a candidate zone where the head(s) is/are likely to be presently positioned; PA1 setting a reference synchronizing frequency to the data recording frequency of the candidate zone; and PA1 updating the zone miss history with current zone miss data. PA1 defining a memory space to stores zone miss history in alternative memory spaces according to the head(s) seek direction at the time of a zone miss; PA1 referencing the one zone miss history that corresponds to the present head(s) seek direction; PA1 estimating a candidate zone where the head(s) is/are most likely to be positioned after the zone miss; PA1 setting a reference synchronizing frequency to the data recording frequency of the candidate zone; and PA1 updating zone miss history in the memory space with the present zone miss data. PA1 measuring the number of mirror marks in a track where the head(s) is/are; and PA1 using the number measured to set a reference synchronizing frequency. PA1 measuring the frequency period of mirror marks in a track where the head(s) is/are; and PA1 using the frequency period measured to set a reference synchronizing frequency.
A rough seek in an optical disk drive will generally get as close as several tracks to several tens of tracks of a target track. If the target track is near the zone boundaries, a "zone miss" phenomena may occur, the head(s) slips out of the target zone after the rough seek to an adjoining zone. The reference synchronizing frequency that is set will probably differ from the data recording frequency of the zone where the head(s) is/are actually positioned, so the optical disk drive circuit cannot read the track address of the track because it cannot synchronize to it. The present track address also cannot be read, so the head(s) is/are effectively lost. There is no way of knowing what the reference synchronizing frequency of the optical disk drive circuit was supposed to be.
The prior art typically uses procedures similar to the following steps to recover from a zone miss:
A prior art zone access method is flowcharted in FIG. 19. In step 1900, a logical address of a seek target sector of a command from a host computer is converted to a physical address. In step 1901, a address for a target zone corresponding to a physical track is indexed from a zone parameter table that has been pre-recorded in a memory within the optical disk system. In step 1902, the current track address is read before doing a rough seek. The difference in distance between the current track address and the target track address is computed to get the head(s) seek distance for a rough seek. In step 1903, the zone address setting is set to equal the address for a target zone. In step 1904, a reference synchronizing frequency is set to the data recording frequency of the target zone obtained in step 1901. In step 1905, the head(s) is/are moved to near the target track in a rough seek according to the seek distance computed in step 1902. In step 1906, a track address identifying where the head(s) is/are after the rough seek is read. In step 1907, the result of the track address reading operation is tested. If it succeeded, the reference synchronizing frequency of the optical disk drive circuit is set to the data recording frequency of the target zone in steps 1909 and 1910, and a fine seek occurs in step 1911. If unsuccessful in step 1907, the drive checks in step 1908 whether or not the present zone setting in the optical disk drive circuit is a maximum zone address for the media. If so, the new zone address setting of the optical disk circuit is set to address zero in step 1913. If the zone address is not a maximum, a new zone address setting is incremented by one, in step 1912. In step 1914, the new zone address setting is tested to see if it has returned to an initial zone address at start of zone access processing. When it does, a media error is reported in step 1916. If not, then in step 1915 the reference synchronizing frequency of the optical disk drive circuit is set to the data recording frequency of the zone that was new in steps 1912 or 1913, and control returns to the track address reading operation of step 1906.
Prior art zone access processing uses simple algorithms, it typically does not take into account any information concerning the conditions under which errors occur, and correct the reference synchronizing frequency usually in only one direction. Consequently, when a head is/are in an adjoining zone because of a zone miss and the zone switching direction is just the opposite, it takes a very long time for the optical disk drive circuit to get to a proper reference synchronizing frequency, and this significantly decreases the processing speed of the drive to a host computer.
There are other means of dealing with zone misses, including one of supplying a new zone in the area between adjacent zones and pre-recording the track address in this new zone with two types of data recording frequencies corresponding to both adjacent zones. (See, Japanese Patent Early Disclosures 1990-189769 and 1990-189742) A method of dividing each zone into a data zone and two buffer zones, one on each side of a data zone, so the buffer zones are sufficiently larger than the estimated seek error is described in Japanese Patent Early Disclosure 1990-183475. These methods are more-or-less effective in cases where comparatively small seek errors have been made. However, track address information must be pre-recorded on the media with two types of frequencies, so mastering of media becomes difficult, thus cost increases are inevitable. Also, the typical phase-locked loop (PLL) circuit becomes complicated, and this also makes higher costs unavoidable. Both methods are not interchangeable with the popular MCAV media now on the market, and is a great impediment in the optical disk drive market. Another problem with these methods is that the data recording area of the media is drastically reduced to make room for the buffer zones, the recording capacity per one media becomes smaller, and the principal advantages of the MCAV media are lost.
The present invention is one that resolves such prior art problems, and its object lies in offering a zone access means that recovers from zone misses quickly, and it is capable of using the MCAV media in the market without sacrificing any of the recording capacity.